This invention relates to magnetic refrigeration systems and more particularly to use of enhanced heat transfer measures in such systems.
It has been known for many years that certain materials exhibit a property known as the xe2x80x9cmagnetocaloric effect,xe2x80x9d hereinafter designated as xe2x80x9cMCE,xe2x80x9d in which the material undergoes a rise and fall in temperature upon being magnetized and demagnetized. Most of the materials which exhibit this property are rare earths and their compounds. This effect is due to changes in entropy as the material is magnetized and demagnetized. These entropy changes are due to varying degrees of order (or alignment) of the magnetic moments of the atoms as these materials are magnetized and demagnetized. Magnetization results in greater order, i.e., decreased entropy, while demagnetization results in increased entropy. Fortunately, these changes are reversible in certain materials. To account for the energy balance, there is an exchange between lattice and magnetic entropy and heat capacity on an atomic level. This phenomenon is most pronounced near the Curie temperaturexe2x80x94that is, the temperature at which a ferromagnetic material becomes para-magnetic. If an MCE material is magnetized while thermally isolated, there is an increase in its temperature known as the adiabatic temperature rise, dTAD.
Use of the MCE as a basis for refrigeration systems has been known for many years. The concept involved is to extract heat from the MCE material while it is magnetized and at an increased temperature, and then to demagnetize the material, causing the temperature to drop. While the temperature is low, heat is transferred from an object to be cooled to the MCE material. The MCE material is then magnetized again, increasing the temperature back to a high level, providing for extraction of heat and rejection of it to a heat sink. Upon repetition of this cycle, heat is transferred xe2x80x9cuphillxe2x80x9d from a low temperature to a higher temperature, which is the definition of refrigeration.
A critical feature required for implementing magnetic refrigeration systems is to ensure that heat flows in the proper direction at the proper time. In effect, a type of one-way thermal switch is needed on both sides of the MCE material so that the heat flows from the MCE material to the heat sink when the MCE material temperature is high but does not reverse and flow back into the MCE material from the heat sink when the MCE material temperature is low, (i.e., demagnetized). Likewise, a one-way thermal switch is needed between the object to be cooled and the MCE material which will allow heat to flow from the object to be cooled to the MCE material when the MCE material temperature is low, but not allow heat to flow back into the object to be cooled from the MCE material when it is at its high temperature (i.e., magnetized).
Early experimentation with this phenonmena resulted in attainment of very low temperature, but at very low capacity, only a small fraction of a watt, for example. Progress has been made over the years since these initial efforts, but practical applications have been limited. One of the limiting factors has been the low value of dTAD, typically five to 15K, which is available to transfer heat through the system. In practical refrigeration, this is known as the xe2x80x9ctemperature lift.xe2x80x9d In the prior art, various means have been used for adding and extracting heat to and from the MCE material, all with a heavy toll on the dTAD. For example, certain designs have used beds of spheres of MCE materials with a complex system of pumps, plumbing, switching valves and a controller to force fluids through these beds, divert the flow through heat exchangers, stop the flow or reverse the flow, as necessary, during each cycle while the MCE material is being magnetized/demagnetized. Other prior art has utilized small, close tolerance gas gaps filled with helium in an attempt to turn the heat flow on and off during the magnetization/demagnetization cycle. These gas gaps are between the moving MCE material and some type of stationary conduction member which then transfers the heat to a heat sink. In order for these gaps to transfer heat, they have to be small. These small gaps have created operational and manufacturing problems. In order to turn these gas gap thermal switches on and off in prior art, the helium gas pressure is repeatedly increased and decreased during each cycle, thus increasing and decreasing the thermal conductance. Other prior art has used heat pipes for transferring of heat in conjunction with the gas gaps. In these embodiments, the heat had to flow through numerous thermal resistances before getting into the heat transfer fluid of the heat pipes, thus using up most of the available dTAD. These heat pipes were embedded in solid copper bars close to the moving MCE material. Heat was conducted from the MCE material, into the gas gap, through the gas gap, into a solid copper member, through the solid copper, through the wall of the heat pipe and into the heat transfer fluid. Such use of heat pipes is examplified by U.S. Pat. No. 4,642,994 wherein disks of MCE material were interleaved with heat pipes. The MCE material, however, was not placed inside of the heat pipe.
Other prior art has used regenerators of MCE material. In these devices heat transfer fluid has to be pumped through these regenerators first in one direction, and then stopped, and then pumped in the opposite direction, all within each cycle. Regenerators have also been used in combination with a displacer unit which cycled the heat transfer fluid back and forth through the regenerator and heat exchangers.
One of the problems presented in developing magnetic refrigerators is that some MCE materials, which exhibit the highest level of performance, have currently proven impossible to form into shapes having optimum heat transfer capability, in particular, fins, tubes, plates or beds of spheres as needed for typical heat exchanger components. These materials are brittle and susceptible to cracking, making them unsuitable for fabrication into conventional refrigerator component shapes.
The present invention is directed to a magnetic refrigerator in which MCE material is intermittently magnetized to produce heating, and heat is extracted from the MCE material while at elevated temperature. Upon demagnetizing the MCE material, its temperature drops. While the temperature is low, heat is transferred from the object to be cooled to the MCE material. The material is then magnetized again, raising the temperature back to its high level, where the heat is again extracted and rejected to a heat sink.
The refrigeration apparatus includes on its high temperature side a thermal diode module comprising a hermetically sealed container having an evaporator end and a condenser end, along with a heat transfer fluid for cycling between evaporation and condensation. At its evaporator end, and within the container, the module has an array of an MCE material disposed in the form of solid, porous matrix derived from finely divided particles and arranged for coming into direct physical contact with the heat transfer fluid during the high-temperature portion of the cycle.
Fabrication of the MCE material into a solid porous matrix obtained from finely divided particles results in a contact surface with a high surface area and significantly enhanced boiling characteristics, causing the process to approach the xe2x80x9cisothermal compressionxe2x80x9d step in the equivalent gas cycle known as the Carnot Cycle, which has the highest efficiency of any known refrigeration cycle. The porous matrix of MCE material produces a boiling surface with a heat-transfer coefficient more than an order of magnitude higher than for a smooth surface. This enhanced surface can result in a heat transfer rate in excess of 200,000 Watts/M2 at a temperature difference between the wall and fluid solution temperature of only one degree K.
The thermal diode module may also include measures to provide enhanced heat transfer at its condenser end such as providing fins of specially shaped cross sections, which drain the condensed heat transfer fluid into grooves, thus maintaining the maximum cold fin area exposed to the vapor being condensed, as described below for the low-side thermal diode module.
The apparatus using the module described above for the high temperature side also includes a low temperature side thermal diode module required to complete the cycle by transferring heat from the object to be cooled to the MCE material. This low temperature thermal diode module is similar to the high-temperature module, except that no MCE material is included. The low temperature side module has its evaporator end thermally coupled to the object to be cooled and its condenser end thermally coupled to the evaporator end of the high temperature module. The low temperature module at its condenser end may include multiple fins which, because of their design and shape, create a pressure gradient due to surface tension, which pumps the condensate off the fins and into grooves, thus continuously exposing the condensing vapor to a maximum area of the cold fins and maintaining a thin meniscus layer of liquid on the fin. This minimizes the thermal resistance due to conduction through the liquid layer.
In order to carry out a refrigeration cycle using the MCE material, the apparatus must include a means for intermittently subjecting the material to a magnetic field. This may be accomplished by providing a magnet which is movable, toward and away from the MCE material or by use of one or more stationary magnets, with the MCE material arranged for being moved in and out of the magnetic field. The magnetic field may be provided by permanent magnets, superconducting magnets or other appropriate means. In a preferred arrangement a plurality of MCE containing modules are disposed on a rotating carrier for being passed into and out of a single magnetic field or a series of magnetic fields. In addition to providing for intermittent magnetization, this rotation enables returning of condensed heat transfer fluid back to the evaporator end by means of centrifugal force.
It is, therefore, an object of this invention to provide a magnetic refrigerator apparatus wherein enhanced heat transfer from magnetized MCE material is obtained with a minimum of temperature difference.
Another object is to provide an improved high temperature side thermal diode module for such apparatus.
Other objects and advantages of the invention will be apparent from the following detailed description and claims appended hereto.